Distributed system technologies are fast developing and the complexity of networked systems increases. For this reason the precise design of distributed systems is necessary, comprising all key aspects. The employment of formal methods is restricted to few aspects such as performance and correctness of distributed algorithms, while the investigation of consistency issues, as they arise e.g. when data is shared between different sites, is still a challenge. In this work, approaches to visual modeDistributed system technologies are fast developing and the complexity of networked systems increases. For this reason the precise design of distributed systems is necessary, comprising all key aspects. The employment of formal methods is restricted to few aspects such as performance and correctness of distributed algorithms, while the investigation of consistency issues, as they arise e.g. when data is shared between different sites, is still a challenge. In this work, approaches to visual modelling and validation of distributed systems are considered focussing on the Unified Modeling Language (UML) as visual modelling language and graph transformation as formal validation domain. To meet the main requirements for distributed system modelling, UML has been extended by a number of profiles and heavy-weight language extensions. For visual reasoning about certain key aspects of distributed systems, the formal calculus of distributed graph transformation has been developed. This calculus supports the formal validation of distributed systems, especially concerning concurrency and consistency issues. To precisely defining the syntax and semantics of a visual modelling language like UML, graphs play a central role, since they are well suited to store the multi-dimensional structures behind visual representations. As a consequence, graph grammars are shown to be a promising technique to define visual languages. Theoretical results for graph transformation can be advantageously used to speed up visual parsing and to show the functional behaviour of model translations. Compared to other approaches to visual language definition, graph grammars allow a fully visual approach which handles all structural aspects visually. The presented approach builds the basis for the precise syntax and semantics definition of visual modelling languages for distributed systems.…